Project Summary/Abstract Metastatic breast cancer (MBC) is a devastating disease that accounts for over 90% of breast cancer mortality and for which there are no current effective treatments. With over 230,000 new breast cancer cases diagnosed annually and accounting for more than 40,000 deaths every year in the United States alone, developing safe and effective treatments for MBC is an utmost priority. Our research program proposes to address this important disease by combining basic and pre-clinical studies that can translate into meaningful clinical outcomes for patients with MBC. Prompted by an increasing understanding of the molecular mechanisms underlying oncogenic dependence and resistance to therapy, the advent of targeted therapies, and, most recently, of immunotherapy, has revolutionized our approach to modern cancer treatment. These advances, together with improved models and novel technologies, open the door to tackling some of the hardest challenges in cancer treatment. We will capitalize on our expertise on signal transduction and pharmacology, as well as on our previous findings on targeted drug resistance and sensitivity, to design safe and effective targeted therapies against MBC. Specifically, we will investigate the role of PTEN and specific PI3K isoforms in metastatic spread using multiple genetically-engineered mouse models (GEMMs) and patient-derived xenografts (PDXs). We will also evaluate the use of combined immunotherapy and targeted PI3K isoform- specific inhibition on syngeneic mouse models of MBC. In addition, we will thoroughly research two recently discovered and promising novel targets, Maternal Embryonic Leucine-zipper Kinase (MELK) and CDK7, which proved to be essential in basal-like or triple negative breast cancer, but dispensable in normal cells, for their role in normal cell physiology and cancer pathogenesis, and for potential targeting in MBC. Importantly, we will invest considerable efforts into researching breast cancer brain metastasis (BCBM), a disease that has been largely neglected due to a lack of clinically relevant models and the difficulty to explore new treatment approaches. To this end, we will use novel orthotopic PDX models of BCBM that faithfully recapitulate genetic and phenotypic characteristics of the original patient samples, to investigate targeted drug combination therapies and resistance. There is a yet unmet need to develop safe targeted therapies against MBC, to thoroughly investigate combined immunotherapy and targeted therapies in breast cancer, and to discover effective treatments against BCBM. We have the experience, expertise and support to carry out these studies, and we are confident that we can make a significant contribution to the field of metastatic breast cancer, and to the many patients and families afflicted by this disease.